Computer not booting

[Cus-Ss816]

When I start the computer I get HP logo follow by screen with red box and message: Invalid signature detected. Check Secure Boot Policy in Setup

I click ok and get a blue box: 

Boot Device Not Found

Please Install an operating system on your hard disk.

Hard Disk - (3FO)

[Hibb1]

Your HDD has either lost its boot partition, or your HDD is faulty.

If you tun the PC on and keep tapping F2, run the diagnostics on the HDD.

What is the result of that?

[Cus-Ss816]

Passed all tests but nothing changed, still getting the same error messages. 

 

[Hibb1]

OK. Go into BIOS (F10) and make sure secure boot is enabled.

If it is and still error, try clearing the secure boot keys.

If still no help, then perhaps you have lost the boot partition for some reason.

You may be able to download and create a recovery media USB from here:

https://support.hp.com/au-en/document/c06162205